Dicationic ionic liquids (DILs) as rapid esterification catalyst of butyric fatty acid

In the presented work, a series of asymmetric dicationic ionic liquids (ADILs) with different alkyl chain length spacer between the two cation nuclei (imidazolium and pyridinium) with chlorine halide anion was designed, synthesized with excellent yield (89, 90 and 88%) and well characterized via different tools of analysis (FT-IR and 1H-NMR spectroscopy and thermal gravimetric analysis; TGA). The synthesized ADILs were examined for potential esterification as recyclable catalysts including the activity of catalytic performance, the reaction conditions justifying. The noted resulted data indicated that the butyric acid was converted perfectly into ester in presence of ADILs with short time of reaction. By completing our studies through the effect of chemical structures, concentrations, time and temperatures, we found that the synthesized Py-6-Imi exhibit the best catalytic performance with 96% as conversion value after 20 min at the ambient temperature (25 °C). The synthesized ADILs also recovered and reused for minimum three rounds without any significant reduction in the catalytic performance. Totally, the usage of ADILs in the esterification process offers lots of benefits such as perfect yield, quick time and environmentally friendly characteristics which make them the optimum sustainable compounds to be achieved in variety of industrial applications.


Synthesis of amphiphilic asymmetric dicationic ionic liquids (ADILs)
As shown in Fig. 1, 1-hexly-2-methyl imidazole (Imi-) was prepared by stirring 0.1 mol of 2-methyl imidazole and potassium hydroxide in 50 ml of acetonitrile.0.1 mol of 1-Chlorohexane was added dropwise to the mixture after complete miscibility.By vagarious stirring for about 3 h, white precipitate of KBr was formed and eliminated by filtration.The filtrate was vaporized and concentrated under vacuum 27 .

Esterification typical procedures
As a typical esterification process as expressed in Fig. 4, add the prepared ADILs (Py-2-Imi, Py-6-Imi and Py-10-Imi) to a stirring mixture of butyric acid and benzyl chloride (1:1), and then triethyl amine was added at room temperature.The reaction mixture was refluxed for 1h at the ambient temperature (25 °C) in presence of 6 wt.% as ADILs ratio during the main experiment.The formation of a white solid precipitate of Et 3 N.HCl is a perfect indication of the reaction progress, and the reaction completion was detected by TLC 25 .Extract the product using 15 × 3 ml of ethyl acetate and dry it with anhydrous Na 2 SO 4 .The ethyl acetate was evaporated and the product was concentrated under vacuum 26 .The obtained product concentrations were determined via the liquid chromatographic technique.The samples were collected at different times (2, 5, 10, 15, 20 and 25 min), at different temperatures (25, 35, 45, 55 and 65 °C)  and different ADILs dosage to study the influence of time, temperature and dosages on the yield so that, 50 ml of the mixture was sampled and extracted using ethyl acetate.The extracted samples were diluted in acetonitrile and analyzed by the HPLC technique by determining the peak area regarding to the calibration curve method.

Ionic liquids recycling process
The importance of ILs recyclability refers to act as sustainable material for other catalysts.After the end of the esterification reaction, the used IL was recovered from the post-reaction filtrate by treating the aqueous layer with ethyl acetate 28 .The solvent was vaporized under vacuum.The structure of the recycled IL was confirmed using FT-IR spectroscopy and reused effectively in new esterification process for 3 times with the same typical esterification process.

Characterizations
The FT-IR bands of the prepared compounds were examined using a Nicolet Ia-10 at scan resolutions of 4000-400/cm and 4 cm and a scan rate of 32 cm/min, in that order. 1 H-NMR spectra were screened using BURKER 1 H-NMR spectroscopy in DMSO-d6 solvent.(400.19MHz and a 5-mm broad-band inverse Z gradient probe).Thermo-gravimetric analysis (TGA) was studied using a thermal analyzer at a heating rate of 10 °C/ min.Samples are heated from ambient temperature up to 600 °C under nitrogen flow.The thermal degradation discussion was revealed at the point of 95% weight loss from the original weight.HPLC (Agilent Technologies 1200 Series HPLC) with a UV detector using a ZORBAX 5 µm, 4.6 × 250 mm NH 2 -column.

FT-IR spectroscopy
The listed FT-IR spectra in Table 2 and Fig. 5 revealed the featured bands at 3442, 3420 and 3425/cm related for N-H stretching vibrations of Py-2-Imi, Py-6-Imi and Py-10-Imi respectively.Also, this could be attributed to the presence of the carbene proton in the form of NH +29 .The characteristic bands of aromatic C-H stretching bands appeared at 3064, 3054, 3123 and 3011/cm for Py-2-Imi, Py-6-Imi and Py-10-Imi respectively.Aliphatic C-H (stretching) bands appeared at 2848, 2920, 2850, 2923, 2854 and 2920/cm respectively 30 .The featured bands of C-C vibrations in imidazole ring appeared at 1630, 1630 and 1627/cm, while the aromatic C=C stretching bands were noticed at 1523, 1526 and 1473/cm for Py-2-Imi, Py-6-Imi and Py-10-Imi respectively.The spectra of C-N stretching modes of vibrations in imidazole rings were detected at 1398, 1380 and 1323/cm.The vibrational bands of the in plane bending appeared at 1192, 1174 and 1175/cm respectively.Moreover, the values of 775, 770 and 752/cm are attributed to the out of plane bending vibrations for the synthesized compounds respectively.The vibrational values of 670, 671 and 703/cm represented the ring deformation out of plane bending for imidazole rings in the synthesized series, respectively.1 HNMR spectroscopy 1 HNMR spectra for the synthesized ADILs and the chemical shifts δ were detected as illustrated in Table 3 and Fig. 6.The highly desheilded protons a, b and c have high δ values because of the drawal action of N. The difference in protons d and e values refers to their direct attachment to N + .For Py-2-Imi the four f protons appeared as triplet at δ value 4.18 ppm.For Py-6-Imi and Py-10-Imi protons f appeared different, where f 1 is highly desheilded more than f 2 because of the pyridinum and imidazolium ring effects.The triplet proton g is more desheilded than the multiplet proton i due to the effect of aromatic ring.18 singlet protons j appeared at    www.nature.com/scientificreports/δ value 1.28, 1.24 and 1.27 ppm respectively, due to their similarity.The aliphatic methyl protons k appeared in the triplet at the lowest δ values.

Thermo-gravimetric analysis (TGA)
TGA curves are experimental studies for the thermal behaviour of the prepared compounds.The synthesized ADILs record high resistance toward the thermal degradation, resulting in high onset temperatures 29,30 .The lower thermal stability was indicated for the longer aliphatic spacer between two cations (imidazolium and pyridinium).Figure 7 confirms that Py-2-Imi, Py-6-Imi and Py-10-Imi are thermally stable and the actually first decomposition steps were noticed at 286, 278 and 270 °C respectively.Higher than these temperatures, TGA indicated an endothermal phenomenon until it reached to maximum mass loss 31,32 .

Catalytic performance of the synthesized ADILs
The esterification processes of butyric acid and benzyl chloride was carried out using the synthesized ADILs and was studied as catalysts under mild conditions.It was clear that the reaction mixture was successfully converted to an ester (benzyl butyrate) with a good yield.Ester formation was confirmed via FT-IR spectroscopy Fig. 8.The carboxylic group characteristic bands were disappeared, and the characteristic carbonyl ester band was observed at 1739/cm.All of the studied ADILs have Cl -as an anion so, the performance activity related directly to the alkyl spacer aliphatic chain length between the two aromatic rings in the cation structure and the effect of moiety.It is well known that aromatic based ILs have high performance activity consequentially, imidazolium cations possessed high activity and acidity 33 because of the effect of acidic hydrogen connected between the nitrogen atoms in the imidazolium ring.The presence of a pyridinium ring in the cation structure increases the aromaticity.Also, by increasing the length of the alkyl chain between the two aromatic rings, the performance activity, Table 4 increased as shown in Py-6-Imi which indicates higher activity than in Py-2-Imi as the two cation become closer, they act as one cation but by increasing the spacer, the effect of two cations becomes clear.In the case of Py-10-Imi the position of the two cations became far away from each other and that may have happened because of the effect of steric bulk being twisted 34 and cause decrease in the performance activity Fig. 9.

The influence of ADILs dosage at the esterification processes
To complete the evaluation of the synthesized compounds, the effect of temperature was investigated on the esterification reaction yield.Py-6-Imi was selected as the best of the synthesized ADILs for these further studies.The esterification processes were carried out using 2, 4, 6, 8 wt.% of the selected ADIL at the ambient temperature (25 °C).The product was collected after 20 min of the esterification reaction.The obtained data are observed in Fig. 10.www.nature.com/scientificreports/By using 2 wt.%, the ester formation was 74%, whereas with 4 and 6%, the yield increased gradually to reach 87 and 96% respectively.By completing the increase of dose (8 wt.%), the yield started to decrease regarding saturation so, 6 wt.% of Py-6-Imi was considered a relatively efficient dosage for high esterification yields, as shown in Fig. 10.

The influence of temperatures at the esterification processes
The effect of temperature was investigated on the esterification reaction yield through carrying out the experiment between 25 and 65 °C temperature ranges by increasing 10 °C in each step.The resulting data are presented in Table 5.
The temperature`s considerable impact on the esterification yield in the presence of Py-6-Imi was studied and observed, as in Fig. 11.At the ambient temperature (25 °C), it was noticed that the total conversion into the ester was recorded after 25 min.By increasing the temperature, the time of esterification decreased 35 to reach the maximum conversion, which reached 5 min at 65 °C.The revealed data illustrate that the use of ADILs in esterification process at mild conditions (temperature range 25-45 °C) enables for significant ester yield to be collected at the same reaction conditions.From the point of using energy to supply heat or remove it, the usage of ADILs as sustainable materials is completely in line with green chemistry approach.

Recycling
Py-6-Imi was selected to evaluate the recyclability of ADILs.The experiments on recycling ADILs were carried out at 25 °C, 6wt.% as the dosage concentration and the ester was collected after 20 min after the esterification reaction.When the reaction was completed, the aqueous layer was treated with ethyl acetate to extract the used Py-6-Imi.The solvent was evaporated and dried under a vacuum.The chemical structure of the recyclable Py-6-Imi was confirmed by using FT-IR and there was no noticeable change in its chemical composition 36 followed by the 1st, 2nd and 3rd recycles as shown in Fig. 12.
The recycled Py-6-Imi was involved in the esterification process and the recycling was repeated for three cycles at the optimum mild conditions 37 .The product was collected after 20 min of the esterification process as illustrated in Fig. 13.It was clear that the recycled Py-6-Imi has almost nearly catalytic performance (91, 93 and 95% as esterification yield) without significant change in its chemical composition.From the resulted data, the product yield slightly decreases (maximum decrease 5%) which is still more than 90%, so we were confirmed that the catalyst is completely active.

Conclusions
In conclusion, this study affords DILs as a new approach for a better conversion in the esterification process.The newly synthesized series were suggested to be a suitable replacement for the conventional catalysts.According to the resulted data, the optimum conditions were found to be 6 wt.% concentration, after 20 min of reaction and at

Figure 9 .
Figure 9.The esterification yield in presence of the synthesized ADILs.

Table 1 .
The chemical structures and physical properties of the synthesized ADILs.

Table 2 .
FT-IR spectrum for the synthesized ADILs.

Table 4 .
The esterification yield in presence of the synthesized ADILs.